This is my .02
It's all about the food and games at a baby shower! With your catering biz you have the food covered. I would build up a very professional looking binder with games, decorating idea, menus, and such (don't foget pictures). Also I would get recommendations from people you have helped in the past! Search the net and find out how much the services typically cost and go from there

I did special event planning as a career. I started through a floral/party production company and then also freelanced for the Convention Center and other businesses/residents.

Since you already own a catering business, I'd add it as an additional service on whatever brochures/business cards you already have.

Plan some events and take some pictures, so you can build a portfolio and get some testimonials.

Drop some business cards off at limo places, photographers, bridal stores, bakeries, etc.

I'd invest some money into advertising. Nothing terribly expensive to start. Can advertise in the yellow pages and the local pennysaver in the beginning.

You can also contact your local chamber of commerce for networking opportunities. If you can afford to join them, I recommend it.

You should decide if you're only planning or executing too. Some people only consult and coordinate, while others are full service and carry through with executing the actual event. (ie. contracting specialists, props, designs, deliveries, etc )

Just my two cents. Without knowing how big or small you're considering I just replied some tidbits.